Yeast on the Rise:
Investigative Study of Fermentation
in the Introductory Biology Curriculum
Steven R. Spilatro, Department of Biology
Marietta College, Marietta, OH 45750
(740) 376-4748
fax (740) 376-4753
spilatrs@marietta.edu
Investigation of fermentation provides opportunities for introductory level students to study
metabolism and develop skills used in experimentation. I describe in this article a simple, inexpensive
apparatus for measuring yeast fermentation and suggestions for its use in investigative studies. This system
is well suited to investigative learning because it allows rapid, reproducible measurements and easy
manipulation of variables that affect fermentation. The lab exercise as provided to the students is included
in this article. We also make available other information about factors that affect fermentation, library
resources and cellular respiration in a Web Resource Page. The most current versions of the lab exercise
and resource page can be located by navigating from the Marietta College Biology Department home page
at www.marietta.edu/~biol.
Quantifying ethanol or CO2 production are two standard approaches to measuring yeast
fermentation. A number of apparatuses for measuring CO2 production that have been previously described
are suitable for classroom use (for example, see Mader, 1994; Reiking et al, 1994; Tatina, 1989;
Yurkiewicz et al, 1989). Many of these systems are poorly suited for investigative studies because of
complexity or awkwardness of instrumentation, lengthy time periods required for data collection, or poor
reproducibility of results. The system described here involves volumetric measurements of yeast CO2
production by recording displacement of a water droplet in a pipette. Although rather traditional in its
approach, the apparatus addresses the above issues and is inexpensive and well suited to investigative
learning activities. Using this system, students can study effects of a wide range of parameters on the
fermentation process, and experiments can be easily replicated within a single lab period. Moreover, the
data generated provide an opportunity to show students how to use computers to calculate fermentation
rates and graph data in presentation format.
We use this lab exercise to help students develop a variety of skills. This is our students’
introduction to experimental design, which is presented in a structured fashion during this and other courses
in our biology curriculum. In this lab exercise, students are given an experimental system and asked to
develop a simple protocol for testing a selected variable. As part of the exercise students also learn how
to use Microsoft Excel to graph data and generate trendlines, write a lab report, use scientific literature, and
make a brief group oral presentation. To provide broader relevance for the topic, yeast fermentation has
been placed within the context of biofuel production. This also allows the students to consider some ethical
issues that arise from the application of science.
Biofuel Fermentation - Page 2
The fermentation apparatus (shown in Figure 3) can be constructed from the following components
readily available from scientific supply companies:
Component
Part #
5cc syringe
BD #301603 (non-luerlock)
1 ml pipette
glass disposable with 0.01 ml gradations
plastic tubing
I.D = 1/8”; O.D. = ¼”; wall = 1/16” (Tygon AACO6007)
The apparatus consists of a syringe and two 1 ml pipettes held together by short lengths of plastic tubing.
We score and snap off the non-calibrated part of the pipettes. The apparatus works with a single pipette;
however, we stack two pipettes to extend the data collection over a 2 ml range. A large number of these
apparatuses can be constructed at minimal cost. Thus, students can be provided with multiple apparatuses
which allows performing concurrent experiments. Yeast mixtures can be prepared in any small container,
although we use 50 ml Erhlenmeyer flasks (Corning #4980) because it allows for easy swirling and mixing
of components.
We use 1 lb bulk packaged ‘rapid rise’ activated dry yeast. This is equivalent to but less expensive
than bakers yeast available in grocery stores. Bulk packaged yeast can be obtained from a local bakery
wholesale supplier or directly from a local bakery or restaurant. We suspend the yeast (80 g per liter) in
water on a magnetic stirrer approximately one hour before lab.
References
Mader, S. S. (1994) Biology Laboratory Manual. Boston MA: W.C. Brown.
Reinking, L.R., Reinking, J,L, & Miller, K. (1994) Fermentation, Respiration & Enzyme Specificity: A
Simple Device & Key Experiments with Yeast. The American Biology Teacher, 56: 164-168.
Tatani, R. (1989) Apparatus & Experimental Design for Measuring Fermentation rates in yeast. The
American Biology Teacher, 51: 35-39.
Yurkiewicz, W.J., Ostrovsky, D.S., & Knickerbocker, C.B. (1989) A Simple Demonstration of
Fermentation. The American Biology Teacher, 51: 168-169.
Acknowledgement
This material is based on work supported by the National Science Foundation under grant No. 9550553
. The government has certain rights to this material.
Biofuel Fermentation - Page 3
BioFuel Production through Yeast Fermentation
Ethanol offers a valuable energy alternative to fossil fuels, which are both nonrenewable and
contribute significantly to atmospheric pollution. Ethanol is often referred to as a ‘biofuel’ since it can be
manufactured by fermenting grains, plant biomass and other organic materials using microorganisms, such
as yeast cells. Ethanol can be used as a fuel directly, but most often it is blended with gasoline (at about
5-10%) to yield gasohol. Since ethanol contains oxygen atoms (Figure 1), gasohol burns ‘cleaner’ than
regular gasoline, reducing emissions of carbon monoxide,
nitrogen oxides and hydrocarbons. Although the raw
materials used to produce ethanol are generally considered Figure 1. The molecular structure of
to be a renewable resource, there are practical and ethical ethanol. The presence of the oxygen
issues that must be considered before society becomes too atom makes ethanol a cleaner burning
dependent upon this energy source (see Ethics of Biofuel fuel then gasoline.
Use below). Nevertheless, ethanol is already making a
significant contribution to world energy needs, and its use will
certainly be increasing.
Currently, ethanol production is measured in the
billions of gallons. Canada and Brazil are major producers
and users of this energy source, much of it derived from
grains such as corn and wheat. To convert grain to ethanol,
the grain is first milled into a fine powder and boiled to liquify the starch. The starch is then enzymatically
broken down to release smaller sugars that can be readily fermented by microorganisms. The ethanol
released during the fermentation process is later distilled from the liquid and further purified. Yeasts are the
type of microorganism most commonly used to ferment grain into ethanol. Yeast have been used in
fermentation processes for millennia, and their use is still the foundation of the baking and beverage brewing
industries. Fermentation is part of the energy producing process called cellular respiration that is shared
by essentially all cells. In this laboratory exercise, you will learn about yeast fermentation and the factors
that affect its rate, and learn some of the basic principles of proper experimental design.
Objectives
This lab exercise has the following objectives:
1. To learn about the use of ethanol as an alternative energy source.
2. To further develop your ability to properly design an experiment and interpret the results.
3. To improve your understanding of cellular respiration and fermentation
Exercise Synopsis
Week 1: read assigned materials; run preliminary fermentation experiment.
Week 2: read assigned materials; plan student-designed experiment; use Excel for graphing.
Week 3: perform student designed experiment; graph and interpret results for week 4.
Week 4: students present experiment, results and interpretations to class.
Biofuel Fermentation - Page 4
Week 1
Before coming to lab for the first week of this lab exercise you should read the following sections
on the bioethics of biofuel production and review the basic experimental methodology. You will graph the
results and calculate fermentation rates, and the class will compare results and discuss sources of
experimental error.
The Ethics of Biofuel Production
Acre / person
Should grains be used as an energy source? Currently, approximately 6% of the U.S. corn crop
is used for ethanol production. There are arguments for and against this practice. A number of indicators
suggest that petroleum supplies will begin to dwindle during the 21st century, and this would seem a
sufficient incentive for us to identify alternative (and ideally renewable) energy sources. Grains and plant
matter (biomass) commonly used in biofuel production are generally considered to be renewable, and thus
ethanol production could help to meet our energy needs. The environmental qualities of ethanol are better
than those of petroleum. The presence of the oxygen atom allows ethanol to burn more cleanly – releasing
less nitrogen oxides, carbon monoxide, hydrocarbons and particulate matter – than gasoline. Gasohol, the
ethanol/gasoline blend, also burns more cleanly than gasoline alone. Thus there are indirect benefits of using
ethanol fuel that cannot be measured directly in energy content and cost.
What is our capacity for ethanol production? Each bushel of corn (approx. 56 lbs) can yield
approximately 2.5 gallons of ethanol. At a current U.S. corn production of 125 bushels per acre, an acre
of farmland would yield enough corn for 310 gallons of ethanol. U.S. gasoline consumption in 1998 was
7 million gallons, and the energy content of a gallon of ethanol is equivalent to approximately 0.7 gallon of
gasoline. A few calculations show that approximately 13,000 mile2 of corn production –an area equivalent
to that of Connecticut– would be required to fully replace the U.S. gasoline consumption. While no one
suggests that ethanol could fully replace petroleum as an energy source, clearly the land area required will
limit our capacity for biofuel production.
However, some people would
suggest that it is wrong to use grain for energy Figure 2. World grain harvested area. This graph is
production instead of food. The World created by dividing the total area used for grain
Watch Institute (Gardner, 2000, 44) reports production by the world population. Data adapted from
that the amount of harvested (farmed) land in Gardner, 2000,45).
the world has declined over the last quarter
0.5
century, while over the same time period
0.45
world population has increased 50%. The
result has been a dramatic decrease in the
0.4
area of harvested land per person (Figure 2).
0.35
This has been partially offset by
0.3
improvements to grain yield per acre;
however, there has been little tangible
0.25
evidence of this improvement for the 70% of
0.2
the world’s population that live in or near
1965 1970 1975 1980 1985 1990 1995 2000
poverty. Proponents of biofuel use also note
Biofuel Fermentation - Page 5
that only the starch from the grain is used to make ethanol, and a bushel of corn used for biofuel production
will also yield 12-15 lbs of protein meal and 3-6 gallons of corn oil. Ultimately, society must decide
whether the virtues of ethanol as an energy source outweigh the potential consequences on available food
supply of using grain for biofuel.
Development of alternative energy sources is not the only way to meet future energy needs. There
are other root causes to potential future energy shortages, including overpopulation, increased energy
consumption by developing nations, and excessive and wasteful energy usage by developed nations such
as the U.S. We are unlikely to meet future energy needs by merely seeking to replace petroleum resources
–other changes in energy consumption habits must also occur.
Gardner, G (2000) Grain Area Shrinks, IN Brown, L.R., Renner, M. and Halweil, B., eds., Vital
Signs 2000, WW Norton and Co., NY, NY, pp 44 - 45.
Figure 3. Respirometer
The procedure for measuring yeast fermentation
As grain is and will continue to be used for biofuel production, it is
appropriate that we should seek to maximize the yield of ethanol. One way to do
this is to study the effects of various factors on the rate of fermentation. There are
a number of ways to measure the rate of yeast fermentation. Since we are
interested in ethanol production, measuring the amount of ethanol produced over
time would be the most direct and useful. Unfortunately, measuring ethanol
released by the yeast cells is too involved for an intro biology lab. However, CO2
production can be measured quickly and accurately, and it affords an acceptable
means of studying the effects of different variables on yeast fermentation.
The experiments you will perform will be carried out using a respirometer
apparatus (Figure 3). The principles of its operation are very simple: the yeast
suspension is placed into a syringe, and the CO2 released by the cells pushes a
small water droplet up a pipet, which allows the volume of CO2 to be measured
at different times over a period of 10 - 20 minutes.
Experimental Procedure
In this week's lab you will study the rate of cellular respiration by yeast cells in
the presence of a solution of glucose present at a concentration of 5% (w/v). The
procedure you will use is described below.
*** Each group member should perform his or her own experiment. ***
1. Weigh out 1.0 g of glucose and place it in a 50 ml flask.
2. Add 10 ml of dH2O to the flask and gently swirl until the carbohydrate is
completely dissolved.
3. Using a 10 ml pipet, transfer 10 ml of the yeast suspension into the
flask.
4. Allow the yeast suspension to incubate for 5 minutes, with occasional swirling.
5. Take up exactly 3 ml of the yeast suspension into a syringe. Invert the syringe and draw 1 ml
of air above the liquid.
Biofuel Fermentation - Page 6
6. Complete the assembly of the respiration apparatus (don't forget the water droplet), and place
it into your test tube rack.
7. As soon as the water droplet reaches the 0 ml mark, begin taking measurements at 2 minute
intervals and record the data in the tables on the following page. You can mark measurements using
either the top or the bottom of the droplet, but be consistent. You may need to decrease the time
interval between measurements to 1 minute or less as the rate of CO2 production increases.
8. Plot the raw data on graph paper, and draw a line through the linear part of the curve
(Figure 4). Using only the linear part of the data calculate the slope of the line as change in
volume ÷ change in time. This yields the rate of respiration in units of ml CO2 / minute.
Table 1. Sample raw data
Time Reading
0 min
3
6
7
8
9
11
0.18 ml
0.25
0.38
0.48
0.55
0.65
0.84
Figure 4. Graphing fermentation raw data. The raw data (Table
1, the actual CO2 volume measurements) is graphed as shown
below. Note that the rate of fermentation is calculated from the
slope of the line drawn through the linear part of the curve.
rate: 0.09 ml/ min
(calculated from graph)
Analysis of results
The fermentation rate obtained by each student should be listed on the board. Each group should also
calculate an average rate, and place ths value on the board as well. These data will be used for a class
discusson of the importance of replication in experimental design and potential sources of experimental
error.
Biofuel Fermentation - Page 7
What are some factors that affect the rate of fermentation in yeast cells?
Before leaving lab the first week, your group will select or be assigned a yeast fermentation parameter
to investigate. The choices are listed below. Groups will be given the opportunity to select a topic, but
some adjustment may need to be made to assure that each group in the class investigates a different topic.
Type of carbohydrate. What are the rates of CO2 production when fermentation is allowed to proceed
in the presence of glucose, fructose, lactose or sucrose?
Concentration of carbohydrate.
yeast fermentation?
What is the effect of glucose over a range of 0% to 2% on the rate of
Concentration of salt. How does varying the salt concentration over a range of 0 - 10% affect the rate
of yeast fermentation?
Tonicity of the environment. Tonicity refers to the concentration of dissolved substances in liquid. What
is the effect of varying the concentration of sorbitol in the fermentation mixture from 0% to 10%?
Ethanolconcentration. What are the effects of varying the ethanol concentration in the fermentation mixture
from 0% to 10% on yeast fermentation?
pH. What is the effect of varying pH in the basic range (from pH 7 to pH 10) or in the acidic range (pH
3 to pH 7) on yeast fermentation?
Temperature. What are the effects on yeast fermentation of temperatures over a range of 0OC to 60OC?
WEEK 2
Before coming to lab you should read the following sections on cellular respiration, experimental
design, and calculating concentration. You are also encouraged to read the Biofuel Lab Internet Resource
Page pertaining to your research topic before coming to lab. During the lab period your group will select
a variable to investigate during week 3, and then plan and turn in a detailed experimental design and
procedure. You will also learn how to use Excel to graph the data you obtained during week 1.
Cellular Respiration and Fermentation
There are many factors that contribute to the economics of biofuel production, including cost of the raw
material, efficiency of the conversion processes, and market value of ethanol. Economic conversion of
starch to ethanol depends to a large extent upon the speed and efficiency of the yeast fermentationprocess,
and biofuel industries are continuously modifying their processes to increase the speed and efficiency of the
yeast fermentation process. The scientists and engineers responsible for improving industrial processes also
must have a detailed knowledge of the cellular processes that are ultimately responsible for the conversion
Biofuel Fermentation - Page 8
of sugar into ethanol. Fermentation is intimately associated with the energy yielding process called cellular
respiration that occurs in essentially all cells, and you also must have some knowledge of this process to
understand the results of this exercise.
What is cellular respiration?
Cellular respiration is a sequence of
enzymatic reactions that extract energy from
molecules, such as carbohydrates and fats, and
make that energy available in the form of ATP.
In most higher organisms, cellular respiration is
an aerobic (oxygen requiring) process. The
three principal stages of cellular respiration are
called glycolysis, Krebs cycle, and the
electron transport (Figure 5).
These
processes are an example of an ‘enzymatic
pathway’, a sequence of chemical changes
catalyzed by a series of different enzymes.
During these processes, electrons are removed
from the organic molecules (a process called
‘oxidation’) and the energy held in the electrons
is eventually transferred to ATP.
The
breakdown of carbohydrates begins with
glycolysis, and the oxidation is completed
during the Krebs cycle. The energy held in the
electrons is subsequently used to produce ATP
during the electron transport process.
Ultimately, the energy-depleted electrons are
deposited on an oxygen atom, the ‘terminal
electron acceptor.’
Figure 5. Overview of cellular respiration. Glycolysis,
Krebs cycle and electron transport proceed in the
presence of oxygen, but fermentation follows glycolysis
when oxygen is lacking.
What is fermentation?
Fermentation is an enzymatic process that allows cells to obtain energy from carbohydrates even in the
absence of oxygen. As described above, in most organisms (and all higher plants and animals) cellular
respiration requires the presence of oxygen (such organisms are said to be aerobic – requiring of oxygen).
If oxygen is not available to serve as the terminal electron acceptor, the electron transport pathway and then
the Krebs cycle will shut down, just as an electric power plant that could not dispose of its waste products
would eventually cease operations. (Each day a coal-burning electric power plant produces tons of flyash
– the powdery ash collected from its smoke emissions – which must be hauled away continuously for
disposal in landfills. If the dump trucks that haul away this waste were to stop running, the power plant
would soon become so inundated with flyash that it would have to shut down.) In a similar manner, if
oxygen is not available to carry away the electrons moving through the electron transport pathway, the
whole process of cellular respiration will grind to a halt.
Biofuel Fermentation - Page 9
For aerobic organisms, the Krebs Figure 6. Overview of the reactions that occur during
cycle and electron transport pathways do fermentation. The NADH produced during glycolysis is
indeed cease to function in the absence of converted back into NAD to allow conversion of pyruvate into
oxygen.
However, glycolysis can ethanol or lactic acid. The result is a net yield of only 2 ATP.
continue if pyruvate is diverted to the
fermentation pathway (Figure 6). The
fermentation pathway provides a way for
the cell to dispose of the pyruvate that
otherwise would build up when the
Krebs cycle is not functioning.
During the fermentation process, the
NADH produced during glycolysis is
converted back to NAD. Thus, the
fermentationpathway utilizes NADH that
the cell would otherwise use to produce
ATP, but it does allow glycolysis to
continue in the absence of O2.
Compared to aerobic respiration, the
energy yield of glycolysis operating
through fermentation is quite low, with a
net yield of only 2 ATP per glucose.
Indeed, this energy yield is too low to sustain the life of higher organisms, and plants and animals cannot
live on glycolysis and fermentation alone. Some cells can function for a while on the energy provided by
fermentation; for example, during strenuous activity, muscle cells continue to function for a short period of
time in this manner. However, the amount of ATP produced anaerobically is too low to support the
tremendous energy demands of our bodies, and this is why higher organisms suffocate in the absence of
oxygen.
How do yeast cells survive via fermentation?
In the presence of oxygen, yeast cells will carry out the complete pathway of cellular respiration.
However, yeast cells can survive and reproduce via fermentation alone. The ethanol and CO2 that are
produced diffuse readily out into the environment surrounding the yeast cells. Single celled organisms can
survive on fermentation alone for several reasons. On one hand, the energy needs of a single cell are much
lower than those of a complex, multicellular organism. Also, yeast cells can import and metabolize
carbohydrates very rapidly – in effect compensating for the inefficiency of fermentation by processing
greater quantities of carbohydrate. As a result, the yeast release great quantities of CO2 and ethanol during
fermentation, which makes these organisms particularly useful to the biofuel industry.
Biofuel Fermentation - Page 10
Some Considerations in Experimental Design
A scientist wishing to increase biofuel ethanol production might evaluate the effects of many different
factors (or variables) on the rate of yeast fermentation. There are many variables that could potentially
affect the rate of yeast fermentation. Indeed, an infinite number of factors (e.g., the alignment of the planets)
might potentially affect the outcome of an experiment; however, a scientist seeking to optimize biofuel
fermentation must use judgement to decide which variables are most likely to significantly affect the
process. Some of these variables would include the type of yeast that is used, type of sugar available,
concentration of the sugar, temperature, and pH of the solution.
A well designed experiment tests the effect of only ONE variable at a time, the one described
in the hypothesis. As you have already learned, a hypothesis is an educated guess of the outcome of the
experiment. Often we have little knowledge by which to formulate a hypothesis, but there is always some
knowledge that can be applied. Some examples of hypotheses would include:
• Yeast fermentation is fastest in the presence of glucose.
• Fermentation rates decrease as temperature is increased.
• The presence of calcium will increase the rate of fermentation.
• etc..
Often, a single experiment will test a range of conditions of the variable. For example, a study of the
effect of temperature may test the effects of three or more different temperatures, and an investigation of
sugar concentration may test the effect of a range of different values.
The results of such experimental tests are always compared to a control. A control is a test that is
identical in every way to the other tests, EXCEPT in the variable that is being tested. Thus, if one
wanted to determine the effect of calcium on the rate of fermentation, the control would be a sample that
lacked calcium but otherwise was the same. Sometimes more than one control test must be included to
rule out other alternative explanations for the results. For example, someone might argue that the CO2
released during this experiment is the result of chemical reactions that do not require living yeast cells (as
was argued by some organic chemists in the early 1800s). What control would you suggest to ‘control for’
(rule out) this possible explanation?
Calculating Concentration
For all of these experiments you must calculate the amount of a material dissolved in the fermentation
mixture. ‘Concentration’ is the most meaningful way to express this quantity. Concentration indicates the
amount of a substance per unit volume of the liquid in which it is dissolved, and units of ‘% w/v’ (percent
weight/volume) or ‘% v/v’ (percent volume/volume) are acceptable for this lab. When the substance being
measured is added as a dry powder, ‘% w/v’ should be used and is calculated by dividing the grams of the
substance added by the final volume of the solution and multiplying by 100:
grams of substance added
% w/v = -----------------------------Final volume (ml) of solution
X 100
For example, in the first week exercise you measured fermentation by yeast cells provided with 1 gram (g)
of carbohydrate dissolved in a total volume of 20 milliliters (ml), which represents a concentration of 5%
w/v.
Biofuel Fermentation - Page 11
When the substance being measured is added as a liquid, ‘% v/v’ should be used and is calculated
by dividing the volume (ml) of the substance added by the final volume (ml) of the solution and multiplying
by 100:
ml of substance added
% v/v = -----------------------------Final volume (ml) of solution
X 100
For example, to make a 5% solution of ethanol in a mixture with a final volume of 20 ml, you would need
to combine 1 ml of ethanol + 19 ml of the other components.
Week 3
Before coming to lab this week you should read the following sections on writing a lab report and
graphing results. During week 3 of this lab exercise you will perform the experiment that you planned.
(The results should be graphed prior to coming to lab next week)
Writing your lab report
You will prepare a lab report for the investigation that you designed and performed during the third
week of this exercise. Each student must write an independent report and prepare his or her own graphs.
The report will include an Introduction, Hypothesis, and Procedures, Results, Discussion and Literature
Cited sections. Further information about writing lab reports is provided in the Document entitled
“Guidelines for Writing Lab Reports” and at the end of this exercise.
Procedures
You will perform the experiment that your group planned last week.
Results
Tables and Figures
Note: all tables and graphs must be formatted (Figure numbers, title, axes labels, etc.) as described in the
Lab Report Guidelines. You should include:
1. Graphs of all raw data, prepared on Excel, with trendlines drawn through the linear region of the data.
To conserve paper, plot the raw data for all conditions of your variable for a given experimental replicate
in a single figure; all lines must be distinguishable and clearly labeled.
2. A table of calculated respiration rates for each replicate and the averages.
Biofuel Fermentation - Page 12
3. A summary graph showing average respiration rates vs your independent variable. You should use the
graph format appropriate to your variable.
Description of Data
1. Describe the raw data for one typical replicate, but only describe any ‘oddities’ in the raw data from
other replicates.
2. Describe the summary graph that presents average respiration rates vs your independent variable.
Graphing Results — Independent vs dependent variables
It is not sufficient in a lab report to include only tables or graphs of the “raw data” (such as that shown
in Table 1 and Figure 3). You must also summarize the calculated results in a suitable graphical form that
reveals relationships between the variable you studied and the rate of fermentation. The results of your
experiments will show the effect of some factor over a range of conditions (e.g., temperatures) on the rate
of yeast fermentation. The results must be presented in an appropriate graphical form that best reveals any
relationships that may exist. To do so, you must select the proper type of graph and plot the data
correctly.
The “independent variable” must be plotted on the X-axis (abscissa) and the “depende nt
variable” must be plotted on the y-axis. The independent variable is the one that you controlled during
the experiment (e.g., time, pH, temperature, or type of carbohydrate). The dependent variable is the value
that was measured or calculated during the experiment. In this case the dependent variable was either CO2
volume (ml) or the calculated respiration rate (ml CO2/min). In a properly designed experiment, the
measured value is dependent upon the value of the independently controlled variable.
The two types of graphs you are most likely to use are a “bar graph” and a “line graph”. A bar graph
is appropriate when there is not a mathematical relationship between the values of the
independent variable. For example, the results of a study of fermentation in the presence of different
types of of carbohydrates should be presented in a bar graph (see Figure 7) because there is no
“mathematical” relationship between ‘types of sugars’.
0.1
0.08
0.06
0.04
0.02
0
Glucose
Lactose
Fructose
Type of Carbohydrate
Galactose
Figure 8. Rate of respiration in presence
of different amounts of sugar.
0.14
Fermentation rate (ml CO
2 /min)
Fermentation rate (ml CO
2 /min)
Figure 7. Fermentation rate in presence
0.12of different types of sugars.
0.12
0.1
0.08
0.06
0.04
0.02
0
0
1
2
3
Sugar concentration (% w/v)
4
Biofuel Fermentation - Page 13
However, a line graph is only used when there is a mathematical or numerical relationship
between the values of the independent variable. Examples include amount of sugar added,
temperature of incubation, incubation time, etc. (see Figure 8). Notice that the data points are clearly
marked and connected by a line. This type of graph helps to reveal trends and relationships in the data that
can be described and interpreted in the report.
Week 4
Prior to coming to lab for week 4 you should read the following section on interpreting results. You
should also consult reference materials about your research topic provided in the library. Each student
should independently graph the results obtained during the previous week on Excel. A copy of your
summary graph will be turned in. During the lab period each group will work together to interpret the
results of the experiment and prepare a 10 minute oral presentation describing the procedure, results and
interpretations.
Discussing and explaining your results
What conclusions and interpretations can be drawn from the results of your experiment? Certainly, the
most important conclusion is whether the results support or contradict the hypothesis. You should always
begin by describing how the results would appear if they support the hypothesis, and then explain how the
actual results compare to these expectations. Remember, your results will never prove that your
hypothesis is correct, but provide support for it. What should you do if your results are ambiguous and do
not allow a basis for a firm assessment? In this case it is important that you explain how the results are
ambiguous. If the results contradict your hypothesis, then you should offer an appropriate alternative
conclusion.
There are other issues that should be addressed as you interpret the results. Your interpretations for
this experiment should assess whether the variable you studied showed an optimum or maximal value. The
effect of quantitative variables (a factor that can be varied by a quantitative amount, e.g., temperature,
amount of sugar, etc.) on a process generally show one of two patterns. When the rates from a series of
experiments where the variable differs quantitatively are plotted, sometimes the rate of the reaction
decreases at values above and below a particular optimal value (Figure 9). Other variables may show
a maximal value, above which the rate remains constant (Figure 10). You can interpret results based upon
your knowledge of cellular and/or enzymatic processes. Why might the variable you studied show an
optimalor maximal pattern? What processes in the cell (properties of membranes, enzyme, pathways, etc)
would account for the results you obtained?
Biofuel Fermentation - Page 14
2/min)
Figure 10. Sample results showing a maximal
value. These data show that the rate of
fermentation does not increase when the value of
the independent variable exceeds 8 (the maximal
value).
0.1
0.08
0.06
0.04
0.02
0
0
2
4
6
8
Value of Independendent Variable
10
Fermentation Rate (ml CO
fermentation rate (ml
CO2 /min)
Figure 9. Sample results showing an optimal value.
These data show that the rate of fermentation is
highest when the independent variable has a value of
5 (the optimal value).
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
0
2
4
6
8
10
12
Value of Independent Variable
\
You should also explain how your results compare with the results of similar studies available in
literature sources. You should consult the Biofuel Lab Internet Resource Page and reference material
available in the library (some are on reserve).
You should also describe the implications of your findings in context of biofuelproduction. How would
you apply your findings if you were actually attempting to improve the process? What are the implications
for a biofuel production process when your variable shows a optimal or maximal value? Do you believe
that your findings afford a means to significantly improve biofuel production? Why or why not?
Even if your results support your hypothesis, you should also propose a future experiment; for
example, you might suggest an experiment to refine your data, or possibly control for another variable. If
your results contradicted your hypothesis or were ambiguous, you might propose an experiment that would
test an alternative hypothesis.
Biofuel Fermentation - Page 15
Data Tables for Yeast Respiration Experiment
Test # ____
time
reading
_0__ min
____ ml
____
____
____
____
____
____
____
____
____
____
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____
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____
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____
____
____
____
____
____
____
____
____
____
rate: ______ ml /min
Test # ____
time
reading
_0__ min
____ ml
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
rate: ______ ml /min
Test # ____
time
reading
_0__ min
____ ml
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
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____
____
____
____
____
____
____
____
____
____
____
____
rate: ______ ml /min
Test # ____
time
reading
_0__ min
____ ml
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
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____
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____
____
____
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____
____
____
____
____
____
____
____
____
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____
____
____
rate: ______ ml/min
Test # ____
time
reading
_0__ min
____ ml
____
____
____
____
____
____
____
____
____
____
____
____
____
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____
____
____
____
____
____
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____
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____
____
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____
____
rate: ______ ml /min
Test # ____
time
reading
_0__ min
____ ml
____
____
____
____
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____
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____
____
____
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____
____
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____
____
____
rate: ______ ml/min
Biofuel Fermentation - Page 16
Data Tables for Yeast Respiration Experiment
Test # ____
time
reading
_0__ min
____ ml
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
rate: ______ ml /min
Test # ____
time
reading
_0__ min
____ ml
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
____
rate: ______ ml /min
Test # ____
time
reading
_0__ min
____ ml
____
____
____
____
____
____
____
____
____
____
____
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____
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____
____
____
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____
____
____
____
____
____
____
____
____
rate: ______ ml /min
Test # ____
time
reading
_0__ min
____ ml
____
____
____
____
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____
____
____
____
____
rate: ______ ml/min
Test # ____
time
reading
_0__ min
____ ml
____
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____
rate: ______ ml /min
Test # ____
time
reading
_0__ min
____ ml
____
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rate: ______ ml/min
Biofuel Fermentation - Page 17
Experimental Plan
Group members:
Each group will turn in a plan describing the experiment you will perform during week 3 of the Biofuel fermentation lab.
The group should work together to plan the experiment and answer the questions. You should feel free to consult with the
lab instructor as necessary.
1. What question are you seeking to answer? What variable will you be testing?
2. What will be your hypothesis?
3. What will be your control(s)?
4. Describe in detail the procedure you will use. (Write it neatly on the back.) Include a description of replicates to be
performed. What range of conditions might be relevant; i.e., range of pHs, temperature, sugar concentration, etc.? Why
was this range selected? Consult with your instructor if necessary when answering these questions.
5. How do you expect the results to appear if your hypothesis is correct? (Draw a hypothetical graph.)